TEMPERATURE & HUMIDITY WHILE HANGING AND FINISHING DRYWALL AND AFTER (Before, During, and After)

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On my recent project of hanging and taping a new house we had to hang all the ceilings before drywall was even brought in for the walls because blown in cellulose was being installed . Just before the insulators were scheduled to come, I visited the job to inspect all the air sealing that was completed by the contractor in the wall cavities and in the attic. After the inspection, I was confident that along the thermal boundary there was an excellent air barrier. The heat was on for several weeks, which allowed the building to dry out. The moisture content of the framing was tested and was at 8 percent, which is excellent. After the insulation was installed and inspected the drywall then was delivered. A few days later we started hanging the walls. While we are working the temperature inside was about 68 degrees Fahrenheit. Fans were runs to help circulate the air and even some windows were partway open to help dump humid air created while taping. This was like a dream job so I better do a great job because I won’t have any excuses.

I live in upstate NY and the temperature this January was a average of 20 degrees Fahrenheit. It has been in the single degrees in the evening . Some days were even below zero. The outside humidity has been 90% or higher. I didn’t expect it to be that high. The humidity inside my jobsite is what I typically worry about. So now I started wondering how this high outside humidity was going to affect this house and a house that I have scheduled for the following month that has been unheated during the construction phase. The reason I was surprised that the humidity was so high is because I had always heard that cold winter air is dry. I know if I am out hiking in the winter exposed skin is often dry and chapped. To better understand temperature and humidity I decided that I needed to study my building science . What I needed to figure out was how temperature, relative humidity, absolute humidity, and dew point affect my drywall jobs. I discovered in my studies the following:

Relative humidity is a ratio of the actual water vapor content of the air to the amount of water vapor needed to reach saturation. Absolute humidity is the mass of water vapor contained in a given volume of air. The relative humidity of a volume of air changes as the temperature changes. The absolute humidity of that volume of air does not.

  • Temperature is easy to understand. I know I can hang and tape a house when it is only 45 to 50 degrees Fahrenheit ,but I know it will dry better and have less potential problems if the temperature is 65 to 70 degrees Fahrenheit. But the question I have is what if the low temperature also had a low humidity and I compared that to a higher temperature with a high humidity. Would the two different scenarios ever reach a point where the drying time would be the same. (Look at chart below). I found a few that would work. At 50 degrees Fahrenheit with 30% humidity and at 70 degrees Fahrenheit with 70% humidity the drying time between coats of compound should be about 26 hours.
  • Relative Humidity: I also understand that when the temperature is hot and the relative humidity is high I sweat easily and have to wait longer for the compound to dry. I am more comfortable working at a lower temperature but also worry about drying. But as long as the humidity is lower at either temperature the compound will dry. Often when discussing best conditions only the temperature is mentioned, this is not sufficient. I have always believed that there are very few perfect drying days for finishing drywall. So to live by the standard of waiting 24 hours between coats of joint compound can lead to problems as temperature and humidity change.
  • Absolute humidity is the actual amount of moisture in the air. If you know the temperature of the air and the relative humidity you can use a psychrometrics chart {see below} to determine the actual amount of water in that air. The actual amount of moisture in the air and how this would affect my drywall work is what I needed to understand better.

Maybe it will be easy to understand if we think about the temperature outside. Today it is 21 degrees Fahrenheit and the sun is shining so from inside the house I am thinking that 21 won’t feel very cold so I decided to go for a walk. It was then that I realized there is a 10 to 15 mph wind. Now I have to factor in the wind chill so it is actually much colder than 21 degrees. It actually feels like 10 degrees – Brrr.

Chart from gypsum association www.gypsum.org

Water vapor in air has a maximum storage capacity, depending on temperature. Warm air can store lots of moisture as vapor in the air, cold air can store only very little.

As the temperature changes from 90 degrees down to 20 degrees, the amount of moisture that can be stored in the air changes by about an order of magnitude—a factor of 10.

Let’s say it’s around freezing outside (32 degrees F) and the relative humidity is 90% , so if I take that air and bring it into the building and heat it up to 70 degrees the amount of moisture in the air stays exactly the same. All I did was heat it. I didn’t add any moisture; I’m just heating it. But because the storage capacity of the air has increased with temperature, the relative humidity drops. And we can get down to less than 25% relative humidity. According to the chart 32degrees at 90% humidity would take 13 days for compound under tape to dry. But at 70 degrees and 25% humidity all you need is 11.5 hours.

So if I want to know what the actual moisture in the air is I need to know both the temperature and the relative humidity. If I put those two pieces together, (temperature and relative humidity)and then refer to a psychrometric chart the absolute humidity can be calculated.

Now I know that understanding how much moisture is in the air is even more important than I thought when finishing drywall. Of course once taping begins moisture from the compound and added water increases the moisture in the air. So ventilation and circulation is important. Maybe even opening a few windows now and then when it is cold outside to dump some of the moisture in the air. This will also work in summer weather if there is more moisture inside the house because high goes to low (high moisture wants to move toward low moisture)

Psychrometric chart

Other Cold Weather Tips

  • Provide heat. When a temporary heat source is used, the temperature should not exceed 95 degrees Fahrenheit (35degrees C) in any given room or area. Heaters should not be allowed to blow directly on wall surfaces. Excessive localized heating can cause joint compound to dry too rapidly resulting in cracking and localized delaminating.
  • Provide sufficient ventilation to ensure normal drying conditions. Certain temporary heaters introduce large amounts of water vapor into the air causing high humidity conditions if not properly ventilated.
  • Joint compound and tape should not be applied to cold or damp surfaces. Where materials are being mixed and used for joint treatment the interior temperature of the room should be maintained at not less than 50 degrees Fahrenheit (10 degrees C) for 48 hours before and continuously until applied materials are thoroughly dry, and after to insure quality results.
  • Protect ready-mixed joint compounds against freezing in storage. Use setting-type compounds to avoid many cold weather related problems.

Copyright 2013-Myron R. Ferguson

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